DE69315145T2 - Multi-layer plastic fuel tank - Google Patents

Description

The present invention relates to a multilayer plastic gasoline tank and, more particularly, to a multilayer plastic gasoline tank which has satisfactory gasoline barrier properties and can be recycled.

In the field of automobile industry, the application of plastics (synthetic resins) for various automobile parts has been eagerly studied with the recent efforts of weight reduction and energy saving. Polyolefin resins have been generally used as plastic materials due to their low cost, high strength, weather resistance and chemical resistance.

High molecular weight polyethylene and high density polyethylene have been proposed as a plastic material for a fuel tank, but it has been reported that its gas barrier properties are not always satisfactory, so that gasoline evaporates, polluting the environment.

A multilayer container having improved gas barrier properties has been proposed in JP-B-61-42625, which consists of a modified nylon resin layer having a polyolefin resin layer laminated on one side thereof via an adhesive layer. (The term "JP-B" as used herein means an "examined Japanese Patent Publication.") The modified nylon resin layer comprises nylon and a molten mixture of a modified copolymer having a maleic anhydride content of 0.05 to 1 wt%, which was prepared by grafting maleic anhydride onto an ethylene-α-olefin copolymer having a degree of crystallinity of 1 to 35% and a melt index of 0.01 to 50 g/10 min. The adhesive layer comprises a modified copolymer having a maleic anhydride content of 0.01 to 1 wt.%, which was prepared by grafting maleic anhydride onto an ethylene-α-olefin copolymer having a degree of crystallinity of 2 to 30% and a melt index of 0.01 to 50 g/10 min.

The polyolefin resin layer used in the above-mentioned multilayer container specifically includes "Novatec BR300", a registered trade name of Mitsubishi Chemical Corp., which is a high density polyethylene having a zero viscosity at 190°C of 1.25 10⁷ poise.

However, the multilayer container using high density polyethylene having a zero viscosity at 190ºC of 1.25 10⁷ poise as a polyolefin resin layer has a disadvantage of poor impact strength, particularly at low temperatures. In addition, due to its poor resistance to deep drawing, thin-walled parts easily appear in the upper part of the multilayer container, and the polyolefin resin layer easily becomes non-uniform due to the poor uniform melt deformability.

Recycling of mold flashes or reclaimed material has been considered to reduce the cost of plastic fuel tanks. However, reusing the flashes or reclaimed material of conventional multi-layer plastic fuel tanks involves a problem in that the resulting fuel tanks have reduced impact resistance.

In order to improve the impact resistance, it has been proposed to knead high-density polyethylene and a part or all of the reclaimed material of the multilayer plastic gasoline tanks (20 to 200 parts by weight per 100 parts by weight of the high-density polyethylene) in an extruder having an L/D ratio of 20 or more and a specially designed screw at an extruding temperature of 200 to 250°C, as described in JP-A-4-47918. (1)The term "JP-A" as described herein means an "unexamined published Japanese patent application").

However, the method described above requires that the kneading temperature be controlled and a specially designed screw be used in a kneading machine. Furthermore, it is difficult to manufacture a fuel tank that maintains superiority in all aspects of impermeability, impact resistance, weather resistance and chemical resistance. Furthermore, due to the reuse of mold flashes or reclaimed material, it is not expected that a reduction in impact resistance will be prevented.

JP-A-4-047 918 describes a multilayer plastic gasoline tank comprising at least one high density polyethylene layer, at least one polyamide layer and at least one of the modified high density polyethylene layers consisting of high density polyethylene modified with an unsaturated carboxylic acid or derivative, and which is interposed between each high density polyethylene layer and each polyamide layer to bond them. The polyamide layer described consists of polyamide-6.

In light of the circumstances described above, the object of the present invention is to provide a multilayer plastic gasoline tank which has satisfactory gasoline barrier properties and which can be recycled without using a special kneading or molding process for producing fuel tanks, while preventing a reduction in impact resistance.

This object was achieved by a multilayer plastic petrol tank comprising (A) a gas-impermeable layer having on at least one side thereof (B) an adhesive layer and further thereon (C) a layer of high density polyethylene, wherein the gas-impermeable layer (A) comprises a modified polyamide composition comprising a mixture of

(1) an α,β-unsaturated carboxylic acid-modified ethylene-α-olefin copolymer prepared by grafting an α,β-unsaturated carboxylic acid or a derivative thereof onto an ethylene-α-olefin copolymer in a grafting ratio of 0.05 to 1% by weight, preferably 0.2 to 0.6% by weight, based on the ethylene-α-olefin copolymer,

and

(2) a polyamide,

includes,

wherein the adhesive layer (B) comprises a resin having adhesiveness to both the gas barrier layer (A) and the high density polyethylene layer (C), and wherein the high density polyethylene layer (C) comprises high density polyethylene having an intrinsic viscosity of 2 to 6 dl/g, a density of 0.940 to 0.970 g/cm3 and a zero viscosity at 190°C of 2.0 10⁷ to 1.0 10⁸ poise (g/cm s).

The multilayer plastic gasoline tank (hereinafter sometimes referred to as a multilayer fuel tank) of the present invention comprises a gas impermeable layer (A) having a high density polyethylene layer (C) laminated on at least one side thereof via an adhesive layer (B).

The gas impermeable layer (A) is described in detail below.

Resins having gas barrier properties are used in the gas impermeable layer (A). Preferably, the ethylene-α-olefin copolymer has a degree of crystallinity of 1 to 35%, more preferably 1 to 30%, and a melt index of 0.01 to 50 g/10 min, more preferably 0.1 to 20 g/10 min. Examples of the α,β-unsaturated carboxylic acid or a derivative thereof include monocarboxylic acids such as acrylic acid and methacrylic acid, their derivatives, dicarboxylic acids such as maleic acid, fumaric acid and citraconic acid, and their derivatives. Preferred examples of the α,β-unsaturated carboxylic acid or a derivative thereof include maleic anhydride.

The modified polyamide composition is described in more detail below.

Examples of the α-olefins in the ethylene-α-olefin copolymer (1) include propylene, butene-1 and hexene-1. The α-olefin is generally copolymerized with ethylene in a ratio of not more than 30% by weight, and preferably 5 to 20% by weight, based on the total amount of the copolymer.

The polyamide (2) generally has a relative viscosity of about 1 to 6. Examples of the polyamide include polyamide obtained by polycondensation of a diamine and a dicarboxylic acid, polyamide obtained by polycondensation of an aminocarboxylic acid, polyamide obtained by polycondensation of a lactam, and copolyamide thereof.

Examples of the diamine include aliphatic, alicyclic or aromatic diamines, such as hexamethylenediamine, decamethylenediamine, dodecamethylenediamine, trimethylhexamethylenediamine, 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, bis- (p-aminocyclohexylmethane), m-xylylenediamine and p-xylylenediamine. Examples of the dicarboxylic acid include aliphatic, alicyclic or aromatic dicarboxylic acids, such as adipic acid, suberic acid, sebacic acid, cyclohexanedicarboxylic acid, terephthalic acid and isophthalic acid. Examples of the aminocarboxylic acid include ε-aminocaproic acid and 11-aminoundecanoic acid. Examples of the lactam include ε-caprolactam and ε-laurolactam.

Specific examples of the polyamide include nylon-6, nylon-66, nylon-610, nylon-9, nylon-11, nylon-12, nylon-6/66, nylon-66/610 and nylon-6/11.

From the standpoint of moldability, a polyamide having a melting point of 170 to 280°C, and particularly 200 to 240°C, is preferred. Nylon-6 is particularly preferred.

The α,β-unsaturated carboxylic acid modified ethylene-α-olefin copolymer is generally blended with the polyamide in an amount of 10 to 50 parts by weight, and preferably 10 to 30 parts by weight, per 100 parts by weight of the polyamide.

Although the method for mixing is not particularly limited, it is preferable to knead them in an extruder, etc. at a temperature of 200 to 280ºC.

The adhesive layer (B) is described in detail below.

The adhesive layer (B) comprises a resin having adhesive properties to both the gas-impermeable layer (A) and the high-density polyethylene layer (C).

Examples of the resin used in the adhesive layer (B) include a modified polyethylene prepared by grafting an unsaturated carboxylic acid or a derivative thereof onto high density polyethylene, preferably in a grafting ratio of 0.01 to 1% by weight based on the high density polyethylene.

The modified polyethylene is described in more detail below.

Preferably, in the above-mentioned modified polyethylene, high-density polyethylene having a density of 0.940 to 0.970 g/cm3 is used. Examples of the high-density polyethylene include ethylene homopolymers and copolymers of ethylene and not more than 3% by weight, preferably 0.05 to 0.5% by weight, of an α-olefin.

Examples of the α-olefin include propylene, butene-1 and hexene-1. Examples of the unsaturated carboxylic acid or a derivative thereof include acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid and anhydride thereof, with maleic anhydride being particularly preferred. The amount of the unsaturated carboxylic acid or a derivative thereof to be grafted is preferably 0.02 to 0.06% by weight.

The high density polyethylene layer (C) is described in detail below.

The high density polyethylene layer (C) comprises high density polyethylene having an intrinsic viscosity of 2 to 6 dl/g, preferably 2.3 to 5.5 dl/g, a density of 0.940 to 0.970 g/cm³, preferably 0.950 to 0.965 g/cm³ and a zero viscosity at 190°C of 2.0 10⁷ to 1.0 10⁸ poise, preferably 2.5 10⁷ to 9.0 10⁷ poise.

Examples of the high density polyethylene include those of various kinds produced by known methods, such as ethylene homopolymers and copolymers of ethylene and not more than 3% by weight, preferably 0.05 to 0.5% by weight, of an α-olefin. Examples of the α-olefin include propylene, butene-1 and hexene-1.

When the intrinsic viscosity or zero viscosity of the high density polyethylene deviates from the respective ranges specified above, the formability such as resistance to deep drawing and uniform melt formability and impact resistance are reduced.

If desired, the high density polyethylene layer may contain additives such as pigments and thermal stabilizers in an amount of not more than 1 part by weight per 100 parts by weight of the high density polyethylene.

The multilayer fuel tank according to the invention is described in detail below.

The multilayer plastic gasoline tank of the present invention comprises the gas barrier layer (A) having a high density polyethylene layer (C) laminated on at least one side thereof via an adhesive layer (B). In particular, a 5-layer structure comprising 3 kinds of layers is preferred, in which a gas barrier layer (A) has a high density polyethylene layer (C) laminated on both sides thereof via an adhesive layer (B).

In constructing the multilayer fuel tank of the present invention, the thickness of the gas impermeable layer (A) is generally 0.01 to 0.5 mm, preferably 0.1 to 0.3 mm; that of the adhesive layer (B) is generally 0.01 to 0.5 mm, preferably 0.1 to 0.3 mm; and that of the high density polyethylene layer (C) is generally 1 to 10 mm, preferably 1.5 to 5 mm.

The multilayer fuel tank according to the invention can be manufactured by a known blow molding process.

For example, the resins for each layer are separately plasticized in multiple extruders, fed into the same die with concentric annular flow paths, laminated in the die while equalizing the respective thicknesses, thereby producing an apparently single-layer parison. The parison is inflated in a mold by applying internal air pressure, thereby bringing it into intimate contact with the mold, and simultaneously cooled, thereby producing a multi-layer fuel tank.

In the manufacture of the multi-layer fuel tank, the flash created during the molding of the tank and/or recovered fuel tanks can be reused as part of the molding material. The flash or recovered material is generally ground into particles or powder in a polymer mill and mixed with the pellets of the high density polyethylene used for the high density polyethylene layer (C). The mixing ratio of the flashing or the reclaimed material is preferably not more than about 60 parts by weight, more preferably not more than 40 parts by weight, per 100 parts by weight of the high density polyethylene. It is also preferred that the flashing and the reclaimed material are dried before mixing.

The multilayer fuel tank of the present invention, even when manufactured by reusing the mold flash and/or reclaimed material, has satisfactory adhesion between the high-density polyethylene layer and the gas barrier layer and does not experience any reduction in strength of the high-density polyethylene layer.

The multilayer fuel tank of the present invention has excellent gasoline barrier properties and is therefore suitable as a fuel tank such as a gasoline tank. Even if mold flashes resulting from the manufacturing step or reclaimed tanks are mixed with the high-density polyethylene, fuel tanks with satisfactory impact resistance can be manufactured.

The present invention will now be illustrated in detail by means of examples.

The following raw materials were used:

(1) High density polyethylene (HDPE):

A: Ethylene-butene-1 copolymer (butene-1 content: 0.2 wt%); intrinsic viscosity: 2.41 dl/g; density: 0.953 g/cm3; zero viscosity: 4.70 10⁷ poise.

B: Ethylene-butene-1 copolymer (butene-1 content: 0.18 wt%); intrinsic viscosity: 4.2 dl/g; density: 0.954 g/cm³; zero viscosity: 4.48 10⁷ poise.

C: Ethylene-hexene-1 copolymer (hexene-1 content: 0.18 wt%); intrinsic viscosity: 5.3 dl/g; density: 0.954 g/cm3; zero viscosity: 9.00 10⁷ poise.

D: Ethylene-hexene-1 copolymer (hexene-1 content: 0.2 wt%); intrinsic viscosity: 2.33 dl/g; density: 0.951 g/cm3; zero viscosity: 1.47 10⁷ poise.

E: Novatec BR300 (a registered trade name of Mitsubishi Chemical Corp.); Intrinsic viscosity: 2.80 dl/g; Density: 0.947 g/cm³; Zero viscosity: 1.25 10⁷ poise.

(2) Modified Polyethylene (APO):

F: Modified polyethylene prepared by grafting maleic anhydride (0.4 wt. %) onto high density polyethylene with a density of 0.960 g/cm3; Melt index (MI): 0.1 g/10 min.

(3) Modified polyamide composition (MPA):

G: Modified polyamide composition prepared by mixing 80 parts by weight of nylon-6 having a relative viscosity of 4.0 and 20 parts by weight of an ethylene-butene-1 copolymer (butene-1 content: 13 mol%; degree of crystallinity: 20%; MI: 3.5 g/10 min) modified with 0.3 wt.% maleic anhydride.

(4) Polyamide (PA):

H: Nylon-6 with a relative viscosity of 3.5 ("Novamid 1020", a registered trade name of Mitsubishi Chemical Corp.).

The physical properties of the raw materials were measured using the following methods:

1) Intrinsic viscosity (η): measured at 130ºC in tetralin.

2) Density: measured in accordance with JIS K6760.

3) Zero viscosity (η₀):

A stress rheometer "RSR-M" manufactured by Rheometrics Co. was used for the measurement. This apparatus enables the measurement of dynamic melt viscosity in the low shear rate range due to creep behavior.

In general, the dynamic viscosity of a molten polymer reaches a steady-state value at a low shear rate (not more than 10³ s⁻¹) and becomes smaller as the shear rate increases. The term "zero viscosity (η₀)" is the steady-state value mentioned above.

The fixation device was a cone-disk type with a diameter of 25 mm and an angle of 0.1 rad between the cone and the disc.

The test samples were prepared by pressing pellets in a molding press into plates with a thickness of about 1 mm. The measurement temperature was 190ºC.

Examples 1 to 3 and Comparative Examples 1 to 3 Manufacturing of the multi-layer fuel tank:

Multilayer fuel tanks with the layer structure shown in Tables 2 and 3 below (5-layer laminate of 3 types of layers) were manufactured as follows.

Resins for each layer shown in Tables 2 and 3 were separately melted in the respective extruders, introduced into the same die with concentric annular flow paths, laminated in the die (die temperature: 235ºC) and coextruded to produce a molten tube (parison).

The resulting parison was clamped in a mold (mold temperature: 20ºC), and air was introduced into the parison at a pressure of 6 kg/cm2, thereby obtaining a multilayer fuel tank with a volume of 60 liters, which had the layer structure and layer thickness shown in Table 2.

The barrel temperature in each extruder was as shown in Table 1. Table 1 Table 2 Table 3

When HDPE E was used as the high-density polyethylene layer, the extruded parison underwent deep drawing while clamped in the mold, which appeared as a thin-walled part in the upper part of the resulting molded part (fuel tank). Furthermore, the high-density polyolefin layer had poor uniformity due to insufficient melt formability.

Strength test of the multi-layer fuel tank:

Each of the multilayer fuel tanks manufactured in Examples 1 to 3 and Comparative Examples 1 to 3 was filled with water or antifreeze. The tank was dropped from the height shown in Table 4 at the temperature shown in Table 4. The strength was evaluated by cracks generated. The results are shown in Table 4. Table 4

Examples 4 and 5 and comparative examples 4 to 6

Each of the multilayer fuel tanks prepared in Examples 1 and 3 and Comparative Examples 1 to 3 was ground in a crusher having a punch hole diameter of 8 mm. The ground material was kneaded in a single-screw extruder or a twin-screw extruder under the conditions shown in Table 5 to obtain chips. A 5-layer fuel tank having a volume of 60 liters comprising 3 kinds of layers was prepared in the same manner as in Example 1 except that the high-density polyethylene layer was made of high-density polyethylene mixed with the resulting chips in an amount of 40 wt% based on the weight of the mixture. Table 5

Strength test of the multi-layer fuel tank:

Each of the multilayer fuel tanks prepared in Examples 4 and 5 and Comparative Examples 4 to 6 was tested for strength in the same manner as in Example 1. The results obtained are shown in Table 6. Table 6

Assessment of material properties:

In Examples 4 and 5 and Comparative Examples 4 to 6, a specific energy Esp in each extruder and the particle diameter of polyamide (NY) were measured for the obtained chips. The obtained results are shown in Tables 7 and 8. Further, the chips were pressed at 230°C to prepare a sample for Izod test, and an Izod strength was measured according to JIS K7110 at 23°C or -40°C. The obtained results are shown in Tables 7 and 8 together with a retention of the Izod strength with respect to an Izod test sample comprising HDPE alone. Table 7 (Single screw extruder)

Remarks:

specific energy: kg h/kW

Izod strength: kg cm/cm Table 8 (twin screw extruder)

As described above, the present invention provides a multilayer plastic gasoline tank which has satisfactory gasoline barrier properties and which can be recycled without requiring special kneading or molding processes, thereby producing fuel tanks while preventing reduction in impact resistance.

Claims (5)

1. A multilayer plastic gasoline tank comprising (A) a gas-impermeable layer having on at least one side thereof (B) an adhesive layer and further thereon (C) a layer of high density polyethylene, wherein the gas-impermeable layer (A) comprises a modified polyamide composition which (1) an α,β-unsaturated carboxylic acid-modified ethylene-α-olefin copolymer prepared by grafting an α,β-unsaturated carboxylic acid or a derivative thereof onto an ethylene-α-olefin copolymer in a grafting ratio of 0.05 to 1 wt% based on the ethylene-α-olefin copolymer, and (2) a polyamide, includes, wherein the adhesive layer (B) comprises a resin having adhesive properties to both the gas-impermeable layer (A) and the high-density polyethylene layer (C), and wherein the high density polyethylene layer (C) comprises high density polyethylene having an intrinsic viscosity of 2 to 6 dl/g, a density of 0.940 to 0.970 g/cm3 and a zero viscosity at 190°C of 2.0 10⁷ to 1.0 10⁸ poise (g/cm s). 2. The multilayer plastic gasoline tank according to claim 1, wherein the ethylene-α-olefin copolymer is an ethylene-α-olefin copolymer having a degree of crystallinity of 1 to 35% and a melt index of 0.01 to 50 g/10 min. 3. A multilayer plastic gasoline tank according to claim 1, wherein the α,β-unsaturated carboxylic acid or a derivative thereof is maleic anhydride. 4. A multilayer plastic gasoline tank according to any one of claims 1 to 3, wherein the adhesive layer (B) comprises a modified polyethylene prepared by grafting an unsaturated carboxylic acid or a derivative thereof onto high density polyethylene. 5. A multilayer plastic gasoline tank according to claim 4, wherein the modified polyethylene was prepared by grafting an unsaturated carboxylic acid or a derivative thereof onto high density polyethylene in a grafting ratio of 0.01 to 1% by weight based on the high density polyethylene.